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Protecting telephone exchanges from lightning surges

Researchers from the IISc have developed new equipment that can ensure better safety of the digital telecom exchanges against lightning surges. The study is carried out by the researchers from the Department of Electrical Engineering, Indian Institute of Science, Bangalore.

Of course, digital telecom exchanges have a built-in system, called protection cards, to protect against voltage and current surges. However, their testing is not carried out in situations that are close to reality of the working environments. The IISc researchers have deviced an equipment that can thoroughly test the protection cards in service like situations. Their findings are published in the journal  IEEE Transactions on Industry Applications.

The telephone wires that run from exchange to our house are exposed to high voltage and high current surges because of multiple reasons like natural lightning, interaction with power transmission lines. A power surge if transmitted to the exchange or the customer premises, can damage the low power telecom equipment.

The surge protection card attempts to limit the over voltage supplied to an electric device either by limiting or diverting the surge to the ground.  Protection cards are designed with different levels of protection called primary and secondary protection. When a high voltage or high current event occurs, the primary protection device absorbs the excess energy instantaneously and bypasses it to the ground. The secondary protection is needed to take care of residual energy left after the primary protection is activated.

The protection cards must be tested before installation in exchanges. Earlier, these cards used to be tested only for high current impulse and high voltage impulse (5kV based on past practice and available literature) with large rate of rise. Commercial test equipment is available for only this type of testing. But this does not cover all possible surge situations. Dr. Reddy says, “In practical situations most of the time the communication line runs along with the power transmission line,  so during occurrence of surges these two lines interacts and as a result of induction, large voltage appears on communication line which appears as oscillatory or a of ring wave in nature.” This means that the protection cards also need to be tested for surges of oscillatory nature, called ring wave surges.

New test equipment developed in IISc can test the protection cards for high voltage surges of oscillatory nature (ring wave surge). The specially fabricated surge generator gives an output voltage of 6 kV and the combination/hybrid wave generator is designed for an output voltage of 8 kV.

This is the first time results of this kind of testing of protection cards is made and published. Researchers suggested that going ahead, all telecom protection cards must be subjected to this kind of testing to ensure better protection and robust systems.

About the authors

Dr. Subba Reddy B is Principal Research Scientist in the Electrical Engineering Department of IISc and a senior fellow, IEEE

Alok Ranjan Verma is a PhD student at the Electrical Engineering Department of IISc and a student member, IEEE

Contact:, 91-080-22932550

About the paper:The paper appeared in the journal IEEE Transactions on Industry Applications. It can be accessed at